John: "Same side same engine different heat or is there something going on here?"
Trying to explain this so it could be understood in this format and time available to me is challenging, but here goes:
Some of the answer to this question will be answered when you remove the gudgeon/wrist pin. I am guessing here, but by looking at the area of the piston in distress that the pin will NOT show a lot of heat distress. I think this was a fairly quick event and not as a result of a long slow soaking build up of heat from extend periods of detonation. The pin might just be the same silver color or a light straw and not show the typical dark blues or purples associated with heavy detonation.
The piston is lacking the distress fore and aft of the pin boss (often called a 4 corner seizure) typically present when the failure is purely detonation. The distress is localized just under the oil ring on the thrust face which tells me that the process happened quickly. I am pretty sure this is a case where the rings didn't seat or "break-in." Just look at the top of the pistons. They are coated with a film of oil.
Check the face of the rings to see if there is a continuous polished surface from one end of the ring gap to the other. There should be no breaks or signs of distress. Do this for all three rings, especially on the piston that didn't seize. If the ring isn't in contact with the cylinder wall you have no heat, or limited heat transfer out of the piston.
If the pre-ignition happened because of valve or spark plug, the damage would have happened so fast as to not give the piston time to heat up and you would not have seen the seizure below the oil ring and the pin would come out bright silver as new. I think it was the build up in heat in piston itself that caused the pre-ignition and it picked the edge of the valve pocket to do it.
So lets leave all this and make a few suggestions to help prevent this. When I was young you could make all sorts of errors during assembly and the high octane rating of the available gasoline would prevent what today would cause this problem. If the rings didn't break-in and some oil got into the chamber it wasn't the problem it is today. So if you are not comfortable with your skills it isn't a bad idea to use a tank of race gas to break in the motor. It gives you a little margin of error until the rings break-in.
That said if the motor shows any signs of oil consumption during break-in I WOULD PERSONALLY install a new set of rings and go through the break-in process again. I have never seen a Triumph "break-in" a set of rings once they failed to do so immediately upon start up. The margin for error is not what it once was!!!
I would match the ring to the coarseness of the honing stones used to prepare the cylinder. A lot of modern motorcycle and automotive machine shops do not have honing stones coarse enough to be used with grey cast iron rings. They will use the fine grit stones required by steel or ductile iron rings.
The typical grey cast iron rings are not lapped round at the factory and are designed to be "honed" by the surface of the cylinder during break-in. This requires a surface you would get using stones of a 150 to 220 grit.
In the day when this bike was made a 150 grit stone was considered finishing and worked well with the grey cast iron rings. Today that same stone would have "roughing" printed on the box and would NEVER be used to prepare a cylinder for ductile iron or steel rings. They require surface finishes of 280 and smoother, almost polished. Few automotive machine shops even have 220 grit, let alone 150 grit stones in the shop.
If modern cylinder finishes of smoother than 220 are used with grey cast iron rings you risk problems unless everything else is right, you have gasoline with enough octane to handle the task and aggressive break-in procedures are used.
Now we are not finished! Modern oil is blended to let the motor get better gas mileage by being very slippery. This isn't something you want when you are trying to break-in a set of grey cast iron rings. In the UK you can buy 10 different brands of "break-in" oil that has little, if any additives (you do want anti-oxidants to prevent the parts from rusting). In the US they make it harder. If you can find it you want to get oil with an SAE rating of CD
or lower. This makes the additive package similar to what was available when the bike was made.
Also many of us have gone to what I like to call "drier" assembly. This is where we lube the wrist pin with assembly lube. Then we wash the cylinder in HOT soapy water and scrub it until white rag comes out absolutely clean. Be patient as this takes time. The I put a small amount of oil on a lint free rag or paper towel and rub the oil on the cylinder walls. I put a drop or so of oil on the thrust faces of the piston and leave the RINGS DRY.
I the use a aggressive break-in. I verify that the oil pump is circulating oil and run the bike up through the gears briskly reaching 4 to 5,000 rpm in each gear. Just the once and follow through with a typical break-in period, being sure not to lug the motor!!!!! These motor like some rpm's!!!!
Now I am begging the question here, but I cannot under emphasize the importance that every thing else be right: carburetor, timing, etc.
And yes, the compression ratio of these "stock" pistons was too high for the existing conditions: Modern low octane fuel, oil in the combustion chamber, lack of heat transfer out of the top of the piston through the rings - some 75% plus of the heat in a piston is transferred to the cylinder walls through the rings!
This hits the high spots as being a mechanic has changed a lot for me in the past 50 years I have been doing it.